There are many situations where it is desirable to monitor the resistance of a power consumer. One example is a heating element in which the resistance of the heating element is an indication of its temperature. It is well known in the art to use a resistance bridge to compare an unknown resistance with a reference resistance. Resistance bridges require three resistances in addition to the one being measured and one of the resistances must carry the full current of the load. This results in a requirement for a resistor capable of conducting the full load current on the side of the bridge containing the load. Also the voltage drop across this resistor represents power used only for the purpose of resistance measurement. Mosfet power transistors are commonly used to control the power to a load circuit. In the prior art both the power transistor and the resistance bridge are required; the power transistor to control the power to the load and the bridge to measure the resistance of the load. Recently, a modified form of the MOSFET power transistor called a SENSEFET has become available in which a very few cells have their sources separated from the remaining sources and brought to a separate connection point outside the transistor. It is well known to use SENSEFET transistors in systems to measure the current being conducted by the transistor.
A general object of this invention is to provide an improved power switch and resistance sensor which overcomes certain disadvantages of the prior art.